Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to a method for casing wellbores drilled within the earth. More particularly, the present invention relates to a method for casing wellbores drilled within the earth by a composite coiled tubing drilling apparatus. More particularly still, the present invention relates to a method for drilling and casing a wellbore with a composite coiled tubing drilling system in a single trip downhole.
2. Background of the Invention
Traditional drilling rigs include large structures that are erected upon land or offshore. The rigs typically support each length of drill string as it is fed into the well and provide rotational motion to a drill bit at the end of the string of drill pipe. Often, when starting a new well, a large diameter drill bit is used for the first several hundred feet of borehole. Once this borehole is complete, the bit is retrieved and a string of metal pipe, known as casing, is placed in the newly drilled borehole. The casing string is slightly smaller in outer diameter than the drilled borehole. Once in the well, the casing is cemented in place and provides a well-defined and fixed reference for subsequent drilling operations. With the first section of casing string installed, a smaller drill bit is lowered through it and is used to drill another, narrower borehole for the next section of casing string to be installed. As each borehole section is drilled, the gage of the drill bit and diameter of the subsequent casing string are reduced until the entire string of casing resembles an extended, inverted telescope. These lengths of casing serve to isolate the drilling and production fluids from the formation surrounding the casing, thereby preventing loss of these fluids into the formation, cross-contamination of the drilling fluids and formation fluids, and degradation of the surrounding formation.
Recent developments in drilling technology have led to the replacement of conventional drill pipe, which is assembled from relatively short lenghts of rigid pipe, with coiled tubing, which is a single length of flexible pipe, typically of steel or a composite. Systems of this type have the ability to operate without conventional pipe-handling equipment, and are capable of drilling much deeper into the earth""s crust and with much more directional capability than was previously achievable. In a composite tubing drilling system, a drilling apparatus is deployed downhole at the end of a long string of composite tubing or hose, the hose being deployed from a large spool on a specialty rig or truck located at the surface. Because no kelly or rotary table is used, all of the mechanical energy to rotate an attached drill bit is created downhole by a downhole drilling motor, with a tractor device being used to maintain the proper amount of weight on bit and torque. As the density of the tubing can be adjusted during manufacture to allow the string to be buoyant in the column drilling fluid, the maximum depth achievable with a drilling system of this type is not limited by the tensile strength of the tubing. Furthermore, because of the relative short tool length and increased flexibility of the composite tubing compared to conventional drill pipe systems, the drilling apparatus is capable of making directional changes with much smaller turning radii than are achievable with rigid drill strings. Additionally, because the composite tubing is preferably manufactured from an electrically insulating material, communications conduits (wire pairs, fiber optic lines) can be incorporated into the sidewall of the tubing during manufacture. Such features enable drillers to send and receive real-time data and commands to and from the drilling apparatus, rather than rely on traditional forms of telemetry.
One drawback to systems of this type is that with the elimination of the conventional drilling rig and the complex directional movement of the composite coiled tubing drilling system, conventional casing strings are not easily deployed into a well created by such a system. Also, because the same apparatus is used to drill an entire well from start to finish, the telescoping casing technique is impractical. Furthermore, because the drilling apparatus may execute complex directional changes along its drillpath, conventional steel casing is not pliable enough to follow such a well contour. To prevent the loss of drilling and production fluids through formation leaching, a casing methodology applicable to wells drilled with composite coiled tubing drilling systems is highly desirable.
The present invention overcomes the deficiencies of the prior art.
The deficiencies of the prior art are overcome using a method that includes delivering an expandable casing string to an uncased borehole coaxially upon a composite coiled tubing drilling string. Once drilling operations are complete in one portion of the borehole, pressure is supplied between the drill string and the coaxially mounted casing string so as to expand the casing string to the full gage of the borehole. Preferably, the outer surface of the casing string includes an adhesive agent that is designed to be activated by the expansion of the casing string against the borehole wall. Alternatively, the mechanical structure of the casing string itself may be configured to prevent the casing string from collapsing once it has been expanded. Once expanded, the casing string is left behind to isolate the well formation from drilling and production fluids that may subsequently flow through the wellbore.
These and other advantages of the present invention will become apparent on reading the detailed description of the invention in conjunction with the drawings.